DELTA AQL (Delta Aquilae). One of the prominent third magnitude
(3.36) stars that help make the extended southern portion of Aquila (the Eagle) southwest of Altair, Delta plays three other roles. It
is also the western-most star of the three-star string that the
Arabs called "Al Mizan," the "Scale Beam" (the others Eta Aql in the middle, Theta to the east).
As such, we could emulate Sagittarius
where the three stars that make the Bow are Kaus Borealis, Kaus
Media, and Kaus Australis (for the
northern, middle, and southern stars). Delta might then carry its
own proper name of "Almizan Occidental," a term never used, so
we'll drop it here. Delta plays an equally strong role as
representing part of the body of a now-defunct constellation called
Antinous (his other stars Theta, Iota,
Kappa, Lambda, Nu, Sigma, and Eta), who was honored as companion to
the Roman emperor Hadrian by placing him as flying with the Eagle.
The pattern, no longer formally recognized, is unusual in being
neither "ancient" nor "modern," but in-between, having been noted
by Ptolemy, appearing on Bayer's Uranometria," but then
finally dropped. The star is fairly close to us, only 50 light
years away. Classed as a mid-temperature (7610 Kelvin) F (F3)
subgiant (one just beginning to become a giant), Delta shines with
the light of 8.2 Suns, almost all of it in
the visual spectrum (there being
no corrections for ultraviolet of infrared), that and temperature
giving it a radius of 1.5 times solar. A projected equatorial
rotation velocity of 88 kilometers per second gives a rotation
period of less than 0.9 days. Then we get a bit of controversy.
The theory of stellar structure and evolution indicates a mass 1.65
times that of the Sun, but show also the Delta Aql is clearly a
dwarf that is somewhat past the middle of its hydrogen-fusing
lifetime of five billion years (half that of the Sun).
Controversies and confusion then accelerate. The star was once
thought to be double from
variations in its spectrum that indicated a companion with a period
of a mere four hours, but further work suggests simple variation
from low-level pulsation. It also seems to be a "Delta Scuti" variable (multi-periodic, low
level variables of class F), but with just one measured period of
1.05 days and a variation of just 0.003 magnitudes (the four day
period not surfacing). But it may yet have a companion, as wobbles
in the motion across the sky suggest one with a period of 3.422
years and an orbital radius of 0.9 Astronomical Units. These
numbers are not at all compatible with the stellar mass, so the
companion remains doubtful. But then maybe there is a real
companion, an 11th magnitude star 110 seconds of arc away. But in
81 years, the two stars separated by 14 seconds of arc, far too
much for orbital motion, rendering the "companion" yet another
line-of-sight coincidence. Factoring in all the data, Delta is
probably a single rather weak example of a Delta Scuti variable.